使用ifstream，ofstream和fstream

Question

1. 试着读一个文件“牛肉”这个词
2. 稍后去编辑文件的内容到用户想要的内容，将存储在字符串中，“line”
3. 文件永远不会显示，当我手动检查时，文本文件为空白。

#include "stdafx.h"
#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main()
{
    string line = { " " };
    ofstream file1;
    file1.open("beef.txt");
    file1 << "beef" << endl;
    file1.close();
    file1.open("beef.txt");

    if (file1.is_open())
    {
        cout << "Enter what you would like to be contained in the file" << endl;
        cin >> line;
        ofstream file1;
        file1 << line << endl;
    }

    return 0;
}

Answer 1

在这个片段中：

if (file1.is_open())
{
    cout << "Enter what you would like to be contained in the file" << endl;
    cin >> line;
    ofstream file1;
    file1 << line << endl;
}

您创建另一个ofstream对象：

ofstream file1;

这会影响您之前创建的那个。而且，你现在正在影子的那个是包含一个指向"beef.txt"的有效文件指针的对象。使用新的内部范围file1还没有指向任何文件，因此写入它不会在"beef.txt"中给你任何结果。

删除它以使用正确的对象：

if (file1.is_open())
{
    cout << "Enter what you would like to be contained in the file" << endl;
    cin >> line;
    file1 << line << endl;
}

Answer 2

std::ofstream仅用于输出，您无法使用它读取输入。

std::ifstream仅用于输入，不能用它来输出输出。

所以，你需要

• 使用单独的std::ofstream和std::ifstream变量：

  int main()
  {
      ofstream out_file;
      ifstream in_file;
      string line;
  
      out_file.open("beef.txt", ios_base::trunc);
      if (out_file.is_open())
      {
          out_file << "beef" << endl;
          out_file.close();
      }
  
      in_file.open("beef.txt");
      if (in_file.is_open())
      {
          getline(in_file, line);
          in_file.close();
  
          cout << "File contains:" << endl;
          cout << line << endl;
      }
  
      cout << "Enter what you would like to be contained in the file" << endl;
      getline(cin, line);
  
      out_file.open("beef.txt", ios_base::trunc);
      if (out_file.is_open())
      {
          out_file << line << endl;
          out_file.close();
      } 
  
      in_file.open("beef.txt");
      if (in_file.is_open())
      {
          getline(in_file, line);
          in_file.close();
  
          cout << "File now contains:" << endl;
          cout << line << endl;
      }
  
      return 0;
  }

• 使用单个std::fstream变量，可以用于输出和输入，具体取决于在调用in时是否指定out和/或open()标志：

  int main()
  {
      fstream file;
      string line;
  
      file.open("beef.txt", ios_base::out | ios_base::trunc);
      if (file.is_open())
      {
          file << "beef" << endl;
          file.close();
      }
  
      file.open("beef.txt", ios_base::in);
      if (file.is_open())
      {
          getline(file, line);
          file.close();
  
          cout << "File contains:" << endl;
          cout << line << endl;
      }
  
      cout << "Enter what you would like to be contained in the file" << endl;
      getline(cin, line);
  
      file.open("beef.txt", ios_base::out | ios_base::trunc);
      if (file.is_open())
      {
          file << line << endl;
          file.close();
      }
  
      file.open("beef.txt", ios_base::in);
      if (in_file.is_open())
      {
          getline(in_file, line);
          in_file.close();
  
          cout << "File now contains:" << endl;
          cout << line << endl;
      }
  
      return 0;
  }

Answer 3

• std::ofstream仅用于输出，您无法使用它读取输入。 std::ifstream仅用于输入，不能用它来输出输出。 所以，你需要

使用单独的std::ofstream和std::ifstream变量：

int main()
 {
ofstream out_file;
ifstream in_file;
string line;

out_file.open("beef.txt", ios_base::trunc);
if (out_file.is_open())
{
    out_file << "beef" << endl;
    out_file.close();
}

in_file.open("beef.txt");
if (in_file.is_open())
{
    getline(in_file, line);
    in_file.close();

    cout << "File contains:" << endl;
    cout << line << endl;
}

cout << "Enter what you would like to be contained in the file" << endl;
getline(cin, line);

out_file.open("beef.txt", ios_base::trunc);
if (out_file.is_open())
{
    out_file << line << endl;
    out_file.close();
} 

in_file.open("beef.txt");
if (in_file.is_open())
{
    getline(in_file, line);
    in_file.close();

    cout << "File now contains:" << endl;
    cout << line << endl;
}

return 0;
    }